How can different torque levels be measured at a single rpm?

Ive been struggling with this for days trying to make sense of a rpm vs torque chart i've seen recently.

The chart states the engine load was measured 100% 70% 40% 0% and at 1000 2000 3000 4000 rpm. How is this done if using a chasis dyno?

I attached the chart for reference.

That's my question, but below i'll ellaborate on my thinking and hopefully there will be a clear error in my understanding....

I think part of my problem is that im unsure what the throttle is doing when these charts are produced using the dyno.

Am I right in thinking that to get the 100% load points over each rpm, the test is started with the engine idling then pushing the throttle to 100%. The RPM will increase rapidly upto redline and the engine torque at each RPM can be estimated working back from wheel RPM and transmission ratio etc. They measure at 4 discrete RPM though, suggesting they might have control over the dyno resitive force?

Good question, A chassis Dyno does in fact have control over the resistance that it is imposing on the power source. Your understanding of the motor is where I perceive the challenge.
At Wide open throttle the torque of an engine is Determined by the cylinder pressure acting on the surface area of the piston and exerting force on the lever of the crank throws. ( to make a broad simplification). This effort will be relatively uniform over the RPM range of the motor. It diminishes as the piston starts to exceed the combustion process at high rpm and due to poor efficiency at quite low rpm.

At part throttle open of any setting you are restricting the air flow and either directly or indirectly the fuel flow. This serves to limit the available cylinder pressures.

A typical Dynamometer run involves a little searching at the low end to find the relative torque point. Then the throttle is advanced to wide open throttle. the machine measure torque applied at RPM to calculate Hp. It is essentially resisting at the maximum effort it can to maintain rpm. As the rpm is increased the Dyno consistently compensates. The resultant recording shows the actual torque developed, the RPM at which the measurement was taken, and the calculated Hp.

It is perfectly possible to do an identical test run at 75% open throttle, 50% open throttle, or any number chosen. The resultant curve would reflect what was produced just the same. The significant variable would be the variations in cylinder pressure due to the restriction of fuel/air mixture admitted. This would be why between 100 and 70 there was a noticeable drop in power but another 30% to 40 showed an apparently disproportionate reduction.